An aircraft which is illuminated by a radar or radar-guided missile can provide an alternate radar target by the use of a physically separate (towed) decoy which is more attractive to the radar or radar-guided missile than that of the aircraft. A more attractive target means generally that the decoy's radar signature is stronger than that of the aircraft and that it contains amplitude and frequency modulation features such that the radar, using advanced signal processing techniques, cannot discriminate between the aircraft and the decoy by exploiting such modulation features (and hence select the aircraft).
At present there are three basic approaches for implementing towed countermeasure decoys. The first approach is the use of an active repeater (amplifier) in the towed body which contains a receive antenna, a transmit antenna (which may be time shared with the receive antenna), an amplifier to increase the decoy's signature power over the aircraft's signature power (when they are received at the radar) and a modulator to provide the appropriate frequency and amplitude modulation so that the radar cannot use modulation characteristics to discriminate between the decoy and the aircraft. In this implementation of the towed decoy, the primary (ac or DC) power source may be contained within the towed body or contained within the aircraft.
The second approach is the use of an active repeater similar to that described above, but housed in the aircraft, and re-radiating the decoy signal from an antenna at the end of a tow-line by propagating the decoy signal down a low loss radio frequency surface wave line (also called a Goubau line) to the re-radiating antenna. This is described in the publication "Surface Waves and Their Applications to Transmission Lines", G. Goubau, Journal of Applied Physics, vol. 21, pp. 1119-1128, November, 1950.
The third approach is the use of an active repeater (similar to that described above) in the aircraft and re-radiating the decoy signal from an antenna in the towed body at the end of the tow-line in a manner similar to that of the second approach above, except the radio frequency line connecting the towed body to the aircraft may or may not be a Goubau line and the towed body will always have an amplifier housed within it to provide additional power to the radiated signature of the decoy.
The primary limitations of the above approaches are that the frequency range over which each of the decoys can respond is limited to the bandwidth of the repeater amplifier, which is typically a travelling wave tube amplifier possessing a bandwidth of one to one and a half octaves.
The power of the decoy signature is limited by the output power of the decoy's power amplifier. This output power limitation simply corresponds to the output saturation power of the travelling wave tube or other output power tube. This limitation means that at sufficiently close range separation between the radar and the aircraft, the output (signature) power of the decoy will attain its maximum (saturation power) while the signature from the aircraft continues to increase as range closes and the signature power of the aircraft received at the radar will ultimately become greater than that of the decoy at sufficiently close range. Under this condition the decoy loses its countermeasure effectiveness.
The tow lines used in the aforenoted implementations (in order that they may transmit from the aircraft to the towed body the required direct current, alternating current or radio frequency power) must be metallic and relatively non-stretchable. Unless a means for artificially slowing the decoy is used during the deployment process with such lines, the line diameter required to withstand the impulse force upon the decoy reaching the end of the line is sufficiently large that line storage is difficult and aerodynamic drag from the line alone is large after deployment. The use of a slowing mechanism or brake also increases the deployment time and reduces decoy effectiveness when deployment must be immediate to counter an imminent threat. The metallic line itself also provides a very large reflective radar signature when viewed side-on which negates the effectiveness of the signature radiated from the antenna at the end of the line for select broadside angles.
On the other hand, a radar target is formed of arrays of passive reflectors contained within a rigid radar-transparent cylinder, having a reflection signature which is similar to that of an airplane. U.K. patents 1,523,268 published Aug. 31, 1978 and 916,067 published Jan. 16, 1963 describe various radar reflective structures in a towable gunnery target, which structures are held within a rigid aerodynamic shell.